Teak refinishing can be done once launched. Stripping of the wood via heat gun is the cleanest method, but you have to be careful not to burn the teak. As far as Cetol vs Armada, that is personal preference. I think Armada has less pigment and appears more translucent than Cetol. Cetol is more readily available. There was an excellent write-up in Practical Sailor on teak treatments that you should review. Cetol and Armada I think are the most user friendly.
Originally, the gloss finish was a two-step process with a clear coat with Cetol and a one part process with Armada, but now I think both companies offer a matte finish and gloss finish in a one-part application. The original Practical Sailor tests showed that the Cetol matte finish was the most durable. You may also consider the new clear finish offered by Armada. I tried to find out from West Marine/Boat US and the manufacturer if the clear finish had similar UV protection but was unable to find any information, so I went with the classic Armada gloss finish. I would follow the manufacturer's recommendations on the can for application.
Usually the best result is obtained by light sanding after heat gun stripping, then I wipe the surface with a clean cloth saturated with acetone. Then I go over the surface with a tac rag. Then apply three coats of finish with a foam throwaway brush. I found no difference using foam brushes as compared to expensive china bristle brushes.
It is important to get the first coat on before the stripped/raw wood is exposed to morning dew. If I could not do this, I would cover the stripped rails with Saran wrap or aluminum foil to keep the dew off until the first coat of varnish is applied. After the first coat is on the wood, the dew is not an issue. Timing is important. Don't apply the finish too late in the day or evening condensation will cloud the finish as it sets up. Of course, you will always be contending with either pollen and/or bugs that seem to be attracted to tacky varnish. If you ignore the occasional bug that lands on your varnish and let it set up completely, they will easily be removed just by wiping with an abrasive towel.
Neither Armada or Cetol require sanding between coats. However, if you want a more varnish-like finish, you can lightly scuff the rails with a maroon Scotchbrite pad. I do not mask the stantions or the fiberglass, and find that if you take your time and immediately wipe off any drips, it is not necessary. In fact, my experience is that if I mask, invariably, the varnish runs under the tape edge and sets up by the time I remove the tape, making more work.